Rogero himself says that the proper and safe way is to do it like this:
3.55 CFW ---> 3.55 OFW ---> 4.30 CFW
These should be done, preferably, via Recovery mode.
I think your current multiMAN supports 4.30 versions because it is resigned for it. Even if it doesn't you can always download another one for 4.30 CFW.
I didn't enable QA Flag or anything while updating my console (guess I was over confident ). All my eboot fixed games work on the new version without any hassle. Yes, multiMAN can read two HDDs at the same time and you can load a game from either.
And you can downgrade to 3.55 kmeaw using the downgraders at any time, but I doubt you would want that. Its much easier to play games now. No waiting for fixes and etc... Its working flawlessly for me.
PS3 Slim 120GB
Rogero 4.30 CFW CEX
1TB TOSHIBA HDD
Playing: Need For Speed: Hot Pursuit, FIFA 13 and Sleeping Dogs
Wish to get: Hitman Absolution, Need For Speed: Most Wanted, Skyrim
Can't wait for: The Last of Us, GTA V